When a user receives a general service, such as audio communication, packet communication or videophone, in a mobile terminal over a mobile communication network, the quality of the service being provided varies depending on the condition of radio waves (e.g., reception field level) from a base station. The user would receive the service at a high quality level if he/she is located near the base station. The quality of the service tends to deteriorate if the user is located far from a base station.
As a technical solution to avoid inconveniences caused by this problem, a typical related art W-CDMA (W-Code Division Multiple Access) system uses a transmission power control system that maintains a required SIR (Signal Interference Ratio) at a predetermined level in order to stabilize communication quality (hereinafter also called a “related art transmission power control system”).
Such a transmission power control system is designed to control an increase or decrease in downlink transmission power (i.e., power transmitted from a base station to a mobile terminal) after DCH (Dedicated Control Channel) synchronization is established, so that the measured quality (M-SIR) of the communication channel over which radio waves are being received will be equal to a target ratio of received signal power to interference power (T-SIR). The system is configured to begin control of the T-SIR value at an initial value (T-SIR) predetermined individually for each bearer (i.e., a communication medium, such as a voice call (AMR), packet (PKT), videophone (UDI=AV) or audio+packet (Multicall)).
More specifically, as shown in FIG. 8, during the control process for T-SIR by the transmission power control system described above, a long segment quality measurement part 82 placed in the outer loop measures the reception quality of a long segment (several hundreds of milliseconds to several seconds). After DCH synchronization is established, the preset initial value outputted from a target quality setting part 84 is updated with a measured reception quality value (BLER: Block Error Ratio, or BER: Bit Error Ratio) for a DPDCH (Dedicated Physical Data Channel) in a DPCH (Dedicated Physical Channel), which is measured by the long segment quality measurement part 82. A comparison determination part 86 determines whether or not the desired BLER is being achieved by comparing the measured reception quality value against the value from the target quality setting part 84, and provides the determination result to a target SIR setting part 18. In this way, the system compensates the T-SIR with slow periodicity, thereby achieving the control of downlink transmission power.
A T-SIR from the target SIR setting part 18 is transmitted to a comparison determination part 26 in a known inner loop (which comprises a back diffusion processing part 20, a Rake reception part 22, an SIR measurement part 24, a comparison determination part 26, and a TPC bit generation part 28). Based on the determination result from the comparison determination part 26, the TPC bit generation part 28 maps a TPC (Transmit Power Control) bit to a DPCCH (Dedicated Physical Control Channel) and transmits it to a base station via DPCCH. The TPC bit is used to control downlink transmission power from the base station.
The purpose of controlling the T-SIR in this manner is as follows: If a user is located near a base station, a service can be provided stably with low transmission power. However, as the user moves away from the base station, it becomes increasingly difficult to maintain the desired communication quality due to deterioration in the radio wave propagation environment, consequently leading to a decreased quality of the service being provided. In order to avoid inconveniences caused by this problem, the T-SIR for a mobile terminal 46 located far from a service area 44 of a base station 42 is heightened to increase downlink transmission power in order to improve, or at least maintain, communication quality and eventually to ensure stable provision of services in a sustainable manner, as shown in FIG. 9.
Literature 1 (Japanese Patent Laying-Open No. 2002-016545) discloses an art in which an apparatus on the receiving side controls transmission power from the transmitting side periodically, so that the SIR or reception power of a received signal from a sender of interest will be equal to a predetermined target reception SIR value or a target reception power value. In this art, the apparatus on the receiving side detects a reception error rate (reception bit error rate or reception frame error rate) of the received signal from the sender of interest, and compares the detected reception error rate against a predetermined target reception error rate on the receiving-side terminal. Based on the result of comparison, the apparatus compensates the target reception SIR value or the target reception power value.
Literature 2 (Japanese Patent Laying-Open No. 2004-207968) discloses a receiving apparatus for a base station, which includes a finger part that outputs an estimated SIR value and the number of GAPs, a decode part that outputs PILOT bit information (the number of error PILOTs) for each of the time slots of a wireless frame, and an SIR threshold offset calculation part which calculates an SIR offset value by adding a predetermined SIR threshold to the number of GAPs outputted from the finger part and the number of PILOTS for each time slot outputted from the decode part, and which is equipped with a synchronization determination part which makes a determination of frame synchronization based on the estimated SIR value outputted from the finger part, the PILOT bit information for the wireless frame outputted from the decode part and the SIR offset value outputted from the SIR threshold offset calculation part.
As described above, the control process performed by a typical related art transmission power control system begins after DCH synchronization is established for each terminal. The initial value of T-SIR used in this control process is a fixed value determined individually for each bearer. Although the initial value is determined by the manufacturer of each system based on many measurement data, the value thus determined is a fixed one and therefore it does not consider actual radio wave conditions and thus communication quality is easily affected by the propagation environment after DCH synchronization is established.
For example, if the radio wave condition, e.g., reception field, is poor when synchronization is established, the M-SIR will not be sufficient and errors will inevitably occur due to poor communication quality, making it difficult to receive services stably. In this situation, the user terminal must continue to request the base station for downlink transmission power to increase the T-SIR for better communication quality until a sufficient level of M-SIR is reached. It requires some time before the user can receive the stable provision of a service.
Conversely, in an environment with a high reception field, the service quality is excessive and the base station must perform control to decrease the T-SIR in order to optimize (increase) the system capacity by lowering downlink transmission power. This increases overhead of the base station with respect to system capacity.
Similarly to the above-described transmission power control system, the related art of Literature 1 controls transmission power from the transmitting side periodically so that the SIR or reception power of a received signal will be equal to a specific predetermined target reception SIR value or target reception power value, and in doing so it compensates the target reception SIR value or the target reception power value based on the result of comparing the detected reception error rate against the predetermined target reception error rate on the receiving-side terminal. However, Literature 1 uses a predetermined target reception error rate for comparison and therefore it potentially holds a technical problem similar to that of the above-described related art transmission power control system.
The art of Literature 2 as well has a similar problem because it uses a predetermined SIR threshold to calculate an SIR offset value.
Developed in light of the above-described circumstances, the purpose of the present invention is to provide a communication control method, a communication control system and its control program that contribute to the provision of communication services stably and so forth.